Galvanic bath, method for producing structured hard chromium layers and use thereof

ABSTRACT

The present invention relates to a galvanic bath, to a process for the precipitation of chromium onto objects, and to the use of the process for the generation of textured hard-chrome layers on machine components. The galvanic bath contains in aqueous solution at least one compound delivering chromium(VI)-ions, and it comprises a) chromium(VI)-ions in an amount that corresponds to 100 to 600 g/ltr of chromic acid anhydride, b) sulfate ions in the form of sulfuric acid and/or of a soluble salt thereof in a molar concentration ratio of chromium(VI)-ions to sulfate ions (SO 4   −2 ) ranging from 90:1 to 120:1, and c) 2-hydroxyethane sulfonate ions in an amount that corresponds to 0.01 to 3.0 g/ltr of the sodium salt.

TECHNICAL FIELD OF THE INVENTION

The present invention is in the field of galvanic coating processes, andgenerally relates to a galvanic bath and to a process for the generationof textured hard-chrome layers on surfaces.

BACKGROUND OF THE INVENTION

It has long been current state of the art to provide objects withsurface coatings by means of galvanic processes in order to impart tothe objects special functional and/or decorative surface properties,such as hardness, corrosion resistance, metallic appearance, luster, orother properties. Galvanic baths that contain nickel or chromium servein technical applications mostly for the generation of especially hard,mechanically resistant layers. In galvanic surface coating processes,the metal to be precipitated is provided in a bath in the form of adissolved salt. The metal is deposited by means of direct current ontothe object to be coated which is connected to a cathode. The object tobe coated consists as a rule of a metallic material. When this is notthe case, and when the object is not electrically conductive, then thesurface of the object can be made conductive, for example, by a thinmetallization.

In certain cases it is requisite or desired that objects that areprovided with a galvanically generated hard-chrome layer present a“rough” surface texture. In decorative coatings, for example, it may bedesired to imprint a matte appearance or an agreeable, non-smooth“feel.” For technical applications, rough hard-chrome layers ortextured-chrome layers have certain functional properties. For example,in machine components which stand in sliding contact with one another,such as, for example, pistons, cylinders, running sleeves, axlebearings, and so forth, rough hard-chrome layers are advantageous, sincethe texture causes lubricant deposits, thus preventing a dry-running. Inthe graphics industry, for sheet-guiding drums in printing presses, forexample, inking rollers and dampening rollers with a rough surface areneeded. Similarly, in the shaping and bending fields, texturally chromedtools can be used in order to impart a textured surface to thework-piece to be processed.

According to conventional techniques, objects with hard-chrome coatingand rough surface texture are obtained by mechanical processing, such asgrinding, sandblasting, spark erosion, etc., or by chemical etchingprocesses before, between, or after the chroming. Such processes,however, are complicated and expensive by reason of the large number ofrequisite different working techniques.

From DE 42 11 881 there is known a galvanic process for applying surfacecoatings to machine components, in which, for example, chromium isprecipitated in textured form. Here by at least one initial, and atleast one subsequent voltage or current impulse, as well as by a certainconducting of the voltage or current function, there is first broughtabout a nucleation on the surface of the machine component, andsubsequently growth of the nuclei of the precipitation material iscaused. The chromium is precipitated in the form of statisticallyuniformly distributed dendritic- or approximately hemispherical-(cap-shaped) raised parts. Another reference, EP 0 722 515, contains afurther development of the process according to DE 42 11 881 in whichthe increase of the electrical voltage or of the current density occursin stages.

The forgoing references disclose the use of conventional galvanic baths.In DE 34 02 554 C2 it is proposed to use a saturated sulfonic acid withat most two carbon atoms and at most six sulfonic acid groups or saltsor halogen acid derivatives thereof to increase the current yield in thegalvanic precipitation of hard-chrome on a work-piece of steel oraluminum alloy from an aqueous non-etching electrolyte containingchromic acid and sulfuric acid. Another document, U.S. Pat. No.5,176,813 discloses a process for the galvanic precipitation of chromiumfrom a galvanic bath with a lead-containing anode in the absence ofmonosulfonic acid, in which the galvanic bath contains chromic acid,sulfate ions and at least one halogenated alkyl polysulfonic acid or itssalt with 1 to 3 carbon atoms.

Known processes in which textured chrome layers are galvanicallygenerated suffer from disadvantages. They demand a complicatedmulti-layer layer-construction, in which, before the textured chromelayer proper is applied, a nickel strike-layer is first applied onto thebase material of the component, then a thicker sulfate nickel layer,followed by the chrome layer, and last of all a covering with afissure-poor hard-chrome layer. These different layers require specific,differently composed galvanic baths and different precipitationconditions, adjusted in each case to the particular layer to bedeposited. The conducting of the process is therefore costly,complicated and, by reason of the necessary working steps, verycost-intensive. Further, there are obtainable via such processes onlylayers with roughness values Rz of up to about 10μ. Moreover, theuniformity of the distribution and the formation of the cap-shapedraised parts is still in need of improvement.

It is a general object of the present invention to substantiallysimplify the generation of textured hard-chrome layers and, inparticular, to make possible textured layers with more uniform surfacetopography and substantially higher roughness values.

SUMMARY OF THE INVENTION

It has now been found that textured hard-chrome layers corresponding tothe foregoing general object can be obtained from a galvanic bath thatcontains at least one chromium (VI)-ion delivery compound and ischaracterized in that it contains the following:

a) Chromium(VI)-ions in an amount that corresponds to 100 to 600 g/ltrof chromic acid anhydride;

b) Sulfate ions in the form of sulfuric acid and/or of a soluble saltthereof in a molar concentration ratio of chromium(VI)-ions to sulfateions (SO₄ ⁻²) of 90:1 to 120:1, and

c) 2-hydroxyethane sulfonate ions in an amount to corresponds to 0.01 to3.0 g/ltr of the sodium salt.

It has been found, surprisingly, that the inventive combination of thecomponents sulfate and 2-hydroxyethane sulfonate results in especiallyadvantageous properties of the chrome bath.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the SEM exposure of the surface of the roller cylindertreated by way of example with the chrome bath of the invention andtexturally chrome-plated according to the process of the invention, withan enlargement of 30:1. The dense and uniform distribution of thespherical cap-shaped raised areas is clearly evident.

FIG. 2 shows a cutout from this zone with an enlargement of 400:1, inwhich the topography of the structure is clarified.

FIG. 3 shows the SEM exposure of a cross section through the layer withan enlargement of 400:1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, the galvanic bath includes chromium(VI)-ions, sulfate ionsand 2-hydroxyethane sulfonate. With regard to chromium (VI)-ions,preferably the galvanic bath according to the invention containschromium(VI)-ions in an amount that corresponds to 200 to 250 g/ltr ofchromic acid anhydride. The compound delivering chromium(VI) ions ispreferably selected from chromic acid anhydride (CrO₃) and/or alkalidichromates such as Na₂Cr₂O₇ and K₂Cr₂O₇. Of the alkali dichromates,K₂Cr₂O₇ is preferred. In an especially preferred embodiment the sourceof chromium(VI)-ions is chromic acid anhydride. In a further embodiment,a part of the source of chromium (VI)-ions includes one or more alkalidichromate(s), preferably potassium chromate. In this embodiment,preferably less than 30%, and more preferably less than 15%, of thechromium(VI)-ions are delivered by alkali dichromate.

With regard to sulfate ions, the molar concentration ratio ofchromium(VI)-ions to sulfate ions in the galvanic bath amountspreferably to 100:1 to 105:1. The usable soluble salts of sulfuric acidare preferably selected from sodium sulfate, potassium sulfate, lithiumsulfate, ammonium sulfate, magnesium sulfate, strontium sulfate,aluminum sulfate and potassium aluminum sulfate. Strontium sulfate isespecially preferred.

The 2-hydroxyethane sulfonate ions contained in the galvanic bath of theinvention can be provided by 2-hydroxyethane sulfonic acid itself or asalt thereof, preferably the sodium salt. In a preferred embodiment, thebath comprises 2-hydroxyethane sulfonate ions in an amount thatcorresponds to 0.07 to 1.5 g/ltr of the sodium salt.

The galvanic chrome bath of the invention can be used in theelectroplating installations ordinarily used in this technology and withthe conventional manners of operation as well as for conventionalcoating purposes and on conventional objects to be coated. Such objectscan be, for example, objects of conductive materials such as metal,especially steel, and metallized nonconducting objects.

The galvanic bath of the invention is purposefully used at temperaturesbetween 30° C. and 70° C. When the galvanic precipitation from such abath is performed at a temperature of ≦50° C. then chrome layers can begenerated with a maximally uniform cup-shaped microstructure, androughness values Rz of up to about 40μ. Such a precipitation ispreferably performed in the temperature range of 40 to 50° C.,preferably between 42 and 48° C., and especially preferably between 44and 46° C.

When the galvanic precipitation from such a bath is performed at atemperature of >50° C., then fissure-poor smooth chrome-layers can begenerated. Such a precipitation is preferably performed in thetemperature range between 51 and 61° C., preferably between 53 and 59°C. and especially preferably between 55 and 57° C.

In this manner, it is directly possible, according to the invention, toproduce from one and the same chrome bath, by variation only of the bathtemperature during the galvanic precipitation, a three-layeredconstruction on the base object. Specifically, the object may beprovided with a first layer comprising a fissure-poor smooth base layer,followed by a textured chrome-layer, and third by a fissure-poor smoothfunctional layer. With the chrome bath of the invention it is possibleto perform the precipitation directly onto materials, such as steel.Galvanic pre-coatings, especially with nickel, are not required.

To deposit a textured hard-chrome layer on to an object, the object,connected to a cathode in a circuit, is brought into the galvanic bathof the invention. There it is sufficient if the object is ground tomeasure. A further surface treatment such as a galvanic pre-coating, isnot required. For an especially uniform coating, it is advantageous tokeep the bath in continuous circulation and/or to keep the object to becoated in continuous rotation in the bath.

The process of the invention can be carried out in the following manner:

In a first step a ground layer is precipitated in the form of a smoothfissure-poor chrome layer at a temperature in the range from 50 to 70°C., preferably from 51 to 61° C. especially preferably from 53 to 59° C.and, most preferably, from 55 to 57° C. The current density can amounthere to up to 50 A/dm². With a precipitation time (TP) of 10 to 15minutes there can be achieved here a base-layer thickness of 6-9μ.Expediently, before the beginning of the precipitation, there is set ina waiting time (TW), during which time the object takes on thetemperature of the bath. This time, depending on the size of the objectand the temperature difference, can amount to 1 to 10 minutes. It isadvantageous before the precipitation to set in an activation step, inwhich the object is positively poled. The current density here canamount to up to 30 A/dm². As time duration TP, 1 to 2 minutes aresufficient. The base-layer obtained has, as a rule, a micro-hardness of800 to 950 HV 0.1.

In the second step there occurs the precipitation proper of the texturedchrome layer from the same bath. For this the bath temperature is to beset at 30 to 50° C., preferably at 40 to 50° C., especially preferablyat 42 to 48° C. and, most preferably, at 44 to 46° C. Also in this stepit is expedient to set in an activation step with the already-mentionedparameters before the beginning of the precipitation. The precipitationoccurs suitably with a current density of 75 to 90 A/dm². With aprecipitation time TP of 10 to 30 minutes there can be achieved here athickness of the textured layer of 14 to 40μ. The textured layerobtained normally has a micro-hardness of 850 to 900 HV 0.1. Thetextured layer obtains a roughness Rz of up to about 40μ.

In the third step, the textured chrome layer is coated with a thin,smooth hard-chrome layer, i.e. the functional layer, again from the samebath. For this, the bath is brought to a temperature in the range of 50to 70° C., preferably of 51 to 61° C., especially preferably of 53 to59° C., and most preferably of 55 to 57° C., and then the precipitationis carried out with a current density of up to 50 A/dm². With aprecipitation time TP of 5 to 15 minutes there can be achieved here alayer thickness of the functional layer of 3 to 9μ. The functional layernormally has a micro-hardness of 1000 to 1050 HV 0.1. By the closingthin hard-chrome layer the roughness of the textured layer is virtuallynot altered. In this step, too, it is again expedient before thebeginning of the precipitation to insert a waiting time TW and anactivation step with the already-mentioned parameters.

In all the precipitating steps it is further advantageous to providebefore the respective precipitating times, a ramp time (TR), in whichthe current density is regulated to the corresponding value. The ramptime TR can in each case amount to 1 to 5 minutes.

The process is distinguished from processes according to the state ofthe art by an especially simple current density management. Thus, forthe generation of a thin, uniform, well textured hard-chrome texturedcoating, it is sufficient, in the respective steps, directly andlinearly to guide the increase and the decrease of the current densityto the respective desired value. Hereby there are not needed theotherwise required, technically expensive, and therefore costly currentand voltage regulating units and their involved programming. In certaincases, however, it can also be favorable and advantageous to regulatethe current in stages to the maximal value, or downward again, instages.

With this procedure there is obtained on the surface of the object atextured hard-chrome layer which is distinguished by an especially denseand uniform distribution of very well formed cup-shaped raised parts.There can be obtained a layer with a peak number of 75 to 100/cm.Depending on the choice of the precipitating conditions, especially inthe step of the textured coating, there can be achieved roughness valuesRz of up to 40μ. The process of the invention can be used to generate achrome layer on components, especially machine components. In apreferred embodiment the process is used to generate a texturedhard-chrome layer on machine components standing in sliding contact withone another, in particular, pistons, cylinders, running sleeves and axlebearings on rollers, drums and cylinders of the graphic industry,especially inking rollers and dampening rollers and on tools.

The following Example is provided to illustrate the invention, butshould not be construed as limiting the scope of the invention.

EXAMPLE

In this Example 100 ltrs of bath containing 20.450 kg of chromic acidanhydride, 2.500 kg of potassium dichromate, 0.550 kg of strontiumsulfate and 3.5 g of 2-hydroxyethane sulfonic acid sodium salt wereprovided. From these there are yielded as concentration values in thebath 222 g/ltr of chromic acid anhydride, 2.2 g/ltr of free sulfate and0.035 g/ltr of 2-hydroxyethane sulfonic acid sodium salt. For thetextured chroming on the example of a roller cylinder made of steel S52as base material from the ground material, there are chosen about thefollowing process parameters:

Precipitation of base layer TW 5.0 min (bath temperature 55 to 57° C.):Activation (30 A/dm²) TR 1.0 min TP 0.5 min Precipitation (50 A/dm²) TR2.0 min TP 10.0 min Precipitation of textured layer TW 0.5 min (bathtemperature 44 to 46° C.): Activation (30 A/dm²) TR 1.0 min TP 0.5 minPrecipitation (75 A/dm²) TR 3.0 min TP 10.0 min TW 3.0 min Precipitation(80 A/dm²) TR 1.0 min TP 10.0 min TW 3.0 min Precipitation (90 A/dm²) TR1.0 min TP 10.0 min Precipitation of functional layer TW 3.0 min (bathtemperature 55 to 57° C.) Activation (30 A/dm²) TR 1.0 min TP 0.5 minPrecipitation (50 A/dm²) TR 2.0 min TP 10.0 min

The structured chrome layer obtained has a roughness Rz of 35 to 40μ,and a peak number of 75-100/cm, with an extremely uniform distributionof very well formed cap-shaped raised parts.

What is claimed is:
 1. A galvanic bath comprising: d) a compounddelivering chromium(VI)-ions, said chromium(VI)-ions being present in anamount that corresponds to 100 to 600 g/ltr of chromic acid anhydride,e) sulfate ions in the form of sulfuric acid and/or of a soluble saltthereof in a molar concentration ratio of chromium(VI)-ions to sulfateions (SO₄ ⁻²) that ranges from 90:1 to 120:1, and f) 2-hydroxyethanesulfonate ions in an amount that corresponds to 0.01 to 3.0 g/ltr of thesodium salt thereof.
 2. Galvanic bath according to claim 1, whereinchromium(VI)-ions are present in an amount that corresponds to 200 to250 g/ltr of chromic acid anhydride.
 3. Galvanic bath according to claim1, wherein said compound is selected from the group consisting ofchromic acid anhydride and an alkali dichromate.
 4. Galvanic bathaccording to claim 1, wherein the molar concentration ratio ofchromium(VI)-ions to sulfate ions ranges from to 100:1 to 105:1. 5.Galvanic bath according to claim 1, wherein the sulfate ions are presentin the form of a compound selected from the group consisting of sulfuricacid, sodium sulfate, potassium sulfate, lithium sulfate, ammoniumsulfate, magnesium sulfate, strontium sulfate, aluminum sulfate andpotassium aluminum sulfate.
 6. Galvanic bath according to claim 5,wherein the sulfate ions are present in the form of strontium sulfate.7. Galvanic bath according to claim 1, containing 0.07 to 1.5 g/ltr of2-hydroxyethane sulfonate, calculated as sodium salt.
 8. A process forproviding a coating on an object, the process comprising galvanicallyprecipitating a chrome-containing coating onto said object from thegalvanic bath according to claim
 1. 9. Process according to claim 8,comprising the following steps: a) Precipitating a ground chrome layerat a temperature of ≧50° C., b) Precipitating a textured chrome layer ata temperature of ≦50° C., and c) Precipitating a functional chrome layerat a temperature of ≧50° C.
 10. Process according to claim 9, whereinindependently from one another the temperature in step a) is in therange between 51 and 61° C.; the temperature in step b) is in the rangebetween 40 and 50° C.; and the temperature in step c) is in the rangebetween 51 and 61° C.
 11. Process according to claim 9, whereinindependently from one another the precipitation of the ground chromelayer in step a) is at a current density of up to 50 A/dm²; theprecipitation of the textured chrome layer in step b) is at a currentdensity of 75 to 90 A/dm²; and the precipitation of the functionalchrome layer in step c) is at a current density of up to 50 A/dm². 12.Process according to claim 9, wherein the rise and/or fall of a currentdensity in steps a), b) and/or c) from the start value to the end valueis linear.
 13. Process according to claim 9, wherein the rise and/orfall of a current density in one or more of steps a), b) and/or c) fromthe start value to the end value is carried out in each case in steps.14. Process according to claim 9, wherein independently from one anotherbefore one or more of steps a), b) and c) an activation is carried outwith a current density of up to 30 A/dm², and wherein said object ispositively polarized.
 15. A process according to claim 9, wherein saidobject is a machine component.
 16. A process according to claim 15,wherein said component is selected from the group consisting of sheetconducting cylinders and drums in the graphic industry.
 17. A processaccording to claim 9, wherein said machine component is selected fromthe group consisting of pistons, cylinders, running sleeves, axlebearings, and rollers in the graphics industry.